Urban areas are of major interest in air pollution monitoring in the United States since most of the population lives in these areas. The most sophisticated and expensive air quality monitoring systems in the United States are those for large cities (and one or two of the largest states) where data collection and analysis are centralized at a single location through the use of telemetry. Online computer facilities provide data reduction.
The three philosophies that can be used in the design of an urban air quality monitoring system are locating sen sors on a uniform area basis (rectilinear grid), locating sensors at locations where pollutant concentrations are high, and locating sensors in proportion to the population distribution. The operation of these systems is nearly identical, but the interpretation of the results can be radically different.
The most easily designed systems are those where sensors are located uniformly on a geographical basis according to a rectilinear grid. Because adequate coverage of an urban area frequently requires at least 100 sensors, this concept is usually applied only with static or manual methods of air quality monitoring.
Locating air quality sensors at points of maximum concentration indicates the highest levels of air pollutants encountered throughout the area. Typically, these points include the central business district and the industrial areas on the periphery of the community. This type of data is useful when interpreted in the context of total system design. One or two sensors are usually placed in clean or background locations, so that the environmental engineer can estimate the average concentration of air pollutants over the entire area. The basis of this philosophy is that if the concentrations in the dirtiest areas are below air quality standards, certainly the cleaner areas will have no problems.
The design of air quality monitoring systems based on population distribution means placing air quality sensors in the most populous areas. While this philosophy may not include all high-pollutant concentration areas in the urban region, it generally encompasses the central business district and is a measure of the air pollution levels to which most of the population are exposed. The average concentrations from this type of sampling network are an adequate description from a public health standpoint. This system design can, however, miss some localized high concentration areas.
Before the system is designed, the system designer and those responsible for interpreting the data must agree on the purpose of air quality monitoring.
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